Ceramic honeycombs and metal honeycombs composed of a stainless steel foil have been widely used as a material of a catalyst carrier for exhaust gas purifying facilities included in automobiles, agricultural machinery, construction machinery, industrial machinery, and the like. Among the above honeycombs, recently, metal honeycombs have been increasingly used because metal honeycombs allow a higher aperture ratio to be achieved and have higher resistance to thermal shock and higher vibration resistance than ceramic honeycombs.
Metal honeycombs have a honeycomb structure formed by, for example, stacking a flat stainless steel foil and a stainless steel foil that has been worked into a corrugated shape alternately. A metal honeycomb including a catalytic material deposited on the surfaces of the stainless steel foils is used in an exhaust gas purifying device. For depositing a catalytic material on the surfaces of the stainless steel foils, commonly, the stainless steel foils are coated with γ-Al2O3 in order to form a wash coat layer on the stainless steel foils and a catalytic material such as Pt or Rh is carried by the wash coat layer.
Since metal honeycombs are exposed to a high-temperature exhaust gas, a stainless steel foil used as a material of metal honeycombs is required to have excellent oxidation resistance. A stainless steel foil used as a material of metal honeycombs is also required to have excellent adhesion (catalyst coating adhesion) to a catalyst coat (i.e., wash coat).
In order to achieve the above-described properties, recent metal honeycombs commonly composed of a high-Al-content ferritic stainless steel foil such as a 20 mass % Cr-5 mass % Al ferritic stainless steel foil or a 18 mass % Cr-3 mass % Al ferritic stainless steel foil. These foils have excellent oxidation resistance because, when the foils are exposed to a high temperature, a protective Al oxide layer mainly composed of α-Al2O3 is formed on the surfaces of the foils. Furthermore, when these foils are subjected to a specific heat treatment, acicular microcrystals, which are referred to as “γ-Al2O3 whiskers” (hereinafter, may be referred to simply as “whiskers”), are formed on the surfaces of the foils, which increase the catalyst coating adhesion. For example, Patent Literature 1 proposes a technique in which the surface of an Al-containing ferritic stainless steel is oxidized by being heated in a low-oxygen atmosphere having a partial pressure of oxygen of 0.75 Torr (99.99 Pa) or less in order to form a whisker-precursor oxide film and the resulting Al-containing ferritic stainless steel is further oxidized in an oxidizing atmosphere in order to grow whiskers on the whisker-precursor oxide film.
FIG. 1 illustrates a result of observing the surface of a ferritic stainless steel foil by a scanning electron microscope, the ferritic stainless steel foil containing, by mass, C: 0.005%, Si: 0.15%, Mn: 0.15%, P: 0.03%, S: 0.002%, Cr: 20.0%, Ni: 0.15%, Al: 5.4%, Cu: 0.1%, N: 0.005%, and the balance being Fe and inevitable impurities which has been subjected to a heat treatment which holds the foil at 900° C. for 30 seconds in a vacuum of 2×10−3 Pa and subsequently subjected to another heat treatment which holds the foil at 900° C. for 24 hours in an oxidizing atmosphere. FIG. 1 confirms the presence of acicular or tabular whiskers formed on the surface of the foil. The formation of whiskers increases the surface area of the foil and accordingly increases the area of contact with a catalyst coat. Further, since the whiskers are acicular or tabular in shape, they also have an effect to anchor the catalyst coat layer. Therefore, forming whiskers on the surface of a ferritic stainless steel foil increases the catalyst coating adhesion.
However, in the above technique proposed in the related art, it is necessary to perform an oxidative heat treatment for a long period of time, that is, about 24 hours, in order to grow whiskers having a sufficient length over the entire surface of the foil. This increases the production cost. A known method for addressing the above issue and forming whiskers in a shorter period of time is a method in which formation of whiskers is promoted by performing a pretreatment.
For example, Patent Literature 2 proposes a method in which blasting is performed as a pretreatment prior to an oxidative heat treatment performed for forming whiskers. It is described in Patent Literature 2 that forming a surface-deformed layer on an Al-containing ferritic stainless steel foil by blasting enables whiskers to be formed on the surface of the foil easily with effect.
Patent Literature 3 proposes a method in which a ferritic stainless steel containing 10% to 30% Cr and 6% to 20% Al is subjected to an heat pretreatment in which the steel is heated to 400° C. to 600° C. in an air atmosphere in order to form θ-Al2O3 on the surface of the steel and the resulting steel is heated to 850° C. to 975° C. in order to grow whiskers. It is described in Patent Literature 3 that forming θ-Al2O3 on the surface of the steel by the heat pretreatment enables whiskers having a high aspect ratio to be uniformly formed on the surface of the steel when a heat treatment is performed in the subsequent step.